There have been 543 jejuno-ileal bypass patients screened for the presence of urinary calculi 1 to 6 years postoperatively. Of these patients 9 per cent have had 1 to 2 calculi during the followup and 3 per cent have had multiple calculi. Ninety-four per cent of the recovered calculi consisted entirely of calcium oxalate. Seven patients had a history of stones before the bypass, 6 of whom have had additional stones postoperatively. To define the conditions associated with stone formation in these patients measurements of serum and urinary oxalate concentration, urinary calcium oxalate saturation, urinary crystal size distribution, and the rates of intestinal oxalate absorption and urinary crystallization have been performed on patients who did and did not have stones postoperatively. On the basis of these studies it appears that the patients in whom stones formed differ from those in whom they did not form only in the rate of urinary crystallization and in the number of large crystal particles present in the urine. Evaluation of current therapeutic modalities in terms of the capability to correct these stone-forming characteristics and to reduce actual calculus formation reveals that the only successful regimen is that which includes an extreme reduction of oxalate ingestion.